Tim Hutcheson wrote:

> > equations than one with two ground planes, termed "stripline".  In his
> case,
> > the two ground planes lower the effective impedance for a given trace
> width.
>
> I never said I was using stripline stackup.  I said in the beginning it was
> a conventional 4-layer board and that "implies" that two microstrip signal
> layers, each referenced to a rail.

Ah, that's why you would need the incredibly thin laminate to get the 50 Ohm
impedance.  I wouldn't sweat it.  The difference between a 50 Ohm and 60 Ohm
line is quite small.  If you put very sharp transients on a long, very low-loss
line,
you get a big reflections.  If you put the same transients on a very short line
that is also a bit lossy, you DON'T see those big transients.  In this
particular
case, where the transient (2 nS Tr) can't fit on the line all at once because
the line is too short, the transient flying back and forth muddles itself, and
it
becomes very spread out.  Therefore, use a trace width and laminate thickness
that is consistent with high quality board manufacturing, and don't worry about
the impedance mismatch, within reason.  Even using 100 Ohm characteristic
impedance traces only 2.5" long will not cause much reflection.  In fact, I
believe you will not be able to SEE the reflection with any scope you care to
try!  It simply can't be very big because of the short line, long Tr.

Jon

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